Dynamical transition of soft porous organic nanocage: The fingerprint of water-regulated ion conduction and viscoelasticity

To understand the importance of water in biomacromolecules and supramolecular materials, the monodispersed 1 nm porous organic nanocage (POC) is studied for their hierarchical condensed structures and relaxation dynamics at different hydration levels. The POCs possess flexible framework and their bulks stay intrinsically in glassy state with well-defined transition temperatures (T_g) and they can be attenuated from 61 to −50 °C with glass-to-rubber transition upon the increasing of hydration levels. Two-level relaxation dynamics can be probed and the temperature dependence of their relaxation time differentiates them as cooperative backbone dynamics (α) and local groups dynamics (β). The dynamical transition of α relaxation is consistent with T_g in hydration dependence. Suggested from MD simulation, water binds to the surface amine groups of POCs and accelerate its local dynamics, leading to the activated backbone dynamics for the broadly tunable visco-elasticity and thermal hysteresis for anti-freeze property. In small-angle X-ray scattering studies, the physical denaturation of POC can be observed as collapsed molecular framework and altered solubility when heated above their T_g in salt solutions. The microscopic understanding inspires the applications of hydrated POCs for elastomers and aqueous lithium electrolyte with high ionic conductivity, chemical stability and mechanical flexibility.